Helical springs used in aircraft systems may be subjected to high cyclical stresses at the natural frequency of the spring. This can occur when the natural frequency is within the operating frequency of the aircraft system. In one example, a particular turbine may operate at frequency of approximately 135 Hz. A helical spring used in a turbine case cooling valve has a natural frequency within the operating frequency of the turbine. As a result, the spring is subjected to vibratory stresses that can contribute to fatigue and ultimately failure of the spring. Failure of the spring can adversely effect the operational efficiency of the turbine.
A turbine case cooling valve regulates the flow of a cooling fluid to the turbine case to maintain a desired clearance between the turbine blades and turbine case. Vibration of the spring at its natural frequency over time can fatigue and break the spring causing a malfunction of the turbine case cooling valve.
Tube or coulomb dampers have been used in conjunction with helical springs to damp and reduce the vibratory response of the spring. However, these dampers apply a frictional force during all helical spring operation causing unwanted wear and hysteresis of the spring.
Helical springs in other aircraft systems and non-aircraft systems may also be subject to the damaging cyclical stresses described above. What is needed is a damped spring assembly capable of reducing resonance of a helical spring and/or its vibratory response to an acceptable level thereby eliminating spring fatigue failures by reducing the cyclical stress on the spring.